Heretofore, electric double-layer capacitors are known, and electric double-layer capacitors of small capacity are used for memory backup powers for personal computers, for assistance for secondary batteries, etc. The electric double-layer capacitor comprises a pair or polarizable electrodes as combined via a separator such as paper or nonwoven fabric put between them to constitute a device, and this serves as an accumulator that produces a small-level electric double-layer capacity to be generated by dipping the device in an electrolyte solution and applying a voltage thereto.
The electric capacity of the electric double-layer capacitor depends on the surface area of the electrode therein, and accordingly, heretofore, the mainstream of the material for the polarizable electrode is activated carbon having a large specific surface area. However, the electric double-layer capacity per the specific surface area of activated carbon is limited, and this is the main reason for the small energy density of the capacitor that comprises activated carbon. In activated carbon having a large surface area, there exist a large amount of graphene sheet edges that cause decomposition of electrolyte solution, and the withstand voltage (charge-discharge voltage) of the capacitor that comprises the activated carbon of the type is generally limited to about 2.5 V.
Accordingly, the withstand voltage of the electric double-layer capacitor comprising an activated carbon electrode is increased up to about 3.3 V according to a special method of, for example, high-temperature hydrogen treatment of electrode material (Patent Documents 1, 2).
However, even the electric double-layer capacitor that comprises such a specifically-processed activated carbon electrode is faced with a problem of capacitance reduction of about 20% in 1000 cycles of charge-discharge repetition at 3.3 V, and it is extremely difficult to attain an excellent cycle characteristic of over 100,000 cycles that is a specific performance of electric double-layer capacitors. In addition, since the activated carbon electrode has a large internal resistance, and the problem must be solved for further capacity increase.
In that situation, carbon nanotubes have high electroconductivity because of their structure, and have a small internal resistance, and therefore, their use for electrodes for electric double-layer capacitors is under investigation, and for example, in Patent Document 3, proposed is a technique of using brush-shaped carbon nanotubes as electrodes of electric double-layer capacitors.
However, conventional carbon nanotubes used as electrodes in electric double-layer capacitors have a bundle structure of such that a few hundreds of tubes are bundled together by van der Waals force, and are therefore problematic in that the carbon surfaces, which are the sites to adsorb electrolyte ions in an electric double-layer capacitor and to determine the electric capacity thereof, are hardly exposed out owing to the bundle formation by the fibers bonding together. Accordingly, the effective specific surface area for electric double-layer formation is small, and the specific surface area obtained according to a nitrogen adsorption method (hereinafter this may be simply referred to “specific surface area”) is a few hundreds m2/g or so. It may be taken into consideration to open the tubes by oxidation treatment or the like to thereby increase the specific surface area, but even in such a case, the specific surface area obtained by a nitrogen adsorption method could be increased up to about 500 m2/g.
In future, use of electric double-layer capacitors is expected in a broad range, for example, for warming up powers for fuel cell automobiles, regenerating powers for hybrid automobiles, powers for heavy industrial machines and robots, etc. For this, further expected are realizations of higher capacitance, higher withstand voltage and more prolonged service life of electric double-layer capacitors.
Patent Document 1: JP-A 2002-362912
Patent Document 2: JP-A 2000-340469
Patent Document 3: JP-A 2003-234254
Non-Patent Document 1: Science, Vol. 306, pp. 1362-1364 (2004)
Non-Patent Document 2: Chemical Physics Letters, 403, pp. 320-323 (2005)
Non-Patent Document 3: Journal of Physical Chemistry B2004, 108, pp.